Farmer-type Ionization Chamber Research Articles

SU-E-T-15: Small and Nonstandard Photon Field Dosimetry Characterization Using Monte Carlo Methods.

To quantify uncertainty reduction in small photon field dosimetry through characterization of ionization chambers and calibration conditions using detailed Monte Carlo methods benchmarked against NIST-traceable measurements. Phase space profiles were obtained using detailed EGSnrc Monte Carlo models for a Varian 6 MV photon linear accelerator, and a NIST-traceable cobalt-60 teletherapy unit. The responses of a farmer-type ionization chamber, two micro-ionization chambers, and one scanning-type ionization chamber were simulated in multiple calibration conditions. Calibration conditions included static field sizes ranging from (10×10) cm squared to (0.5×0.5) cm squared for the 6 MV and cobalt-60 beam qualities and an additional dynamic IMRT plan for the 6 MV beam quality. Calibration conditions also consisted of ionization chambers placed in a standard water phantom and in a specially designed acrylic phantom. Tolerance limits on the calibration conditions were investigated. All models were benchmarked against measured beam quality data, including ionization chamber beam quality correction factors for the standard absorbed dose to water cobalt-60 calibration coefficient. The majority of the simulated small field response values fell within the uncertainty of the measured values. A database was created for several proposed small field calibration conditions to provide comparisons with the Co-60 standard reference conditions. The database includes the small field calibration conditions' beam quality correction factors, tolerances, and dose calibration uncertainties. The characterization of multiple calibration conditions provided an improved understanding of how the cobalt-60 ionization chamber absorbed dose to water calibration coefficient from an ADCL can optimally be applied to small and nonstandard field calibrations to reduce the associated dose uncertainty. The developed methodology will contribute to future research of other small field radiotherapy modalities. The resulting database provides support for future recommendations on the implementation of small and non-standard field calibration protocols.

Non-reference condition correction factor kNR of typical radiation detectors applied for the dosimetry of high-energy photon fields in radiotherapy

According to accepted dosimetry protocols, the “radiation quality correction factor”kQ accounts for the energy-dependent changes of detector responses under the conditions of clinical dosimetry for high-energy photon radiations. More precisely, a factor kQR is valid under reference conditions, i.e. at a point on the beam axis at depth 10 cm in a large water phantom, for 10×10 cm2 field size, SSD 100 cm and the given radiation quality with quality index Q. Therefore, a further correction factor kNR has been introduced to correct for the influences of spectral quality changes when detectors are used under non-reference conditions such as other depths, field sizes and off-axis distances, while under reference conditions kNR is normalized to unity. In this paper, values of kNR are calculated for 6 and 15 MV photon beams, using published data of the energy-dependent responses of various radiation detectors to monoenergetic photon radiations, and weighting these responses with validated photon spectra of clinical high-energy photon beams from own Monte-Carlo-calculations for a wide variation of the non-reference conditions within a large water phantom. Our results confirm the observation by Scarboro et al. [26] that kNR can be represented by a unique function of the mean energy Em, weighted by the spectral photon fluence. Accordingly, the numerical variations of Em with depth, field size and off-axis distance have been provided. Throughout all considered conditions, the deviations of the kNR values from unity are at most 2% for a Farmer type ion chamber, and they remain below 15% for the thermoluminescent detectors LiF:Mg,Ti and LiF:Mg,Cu,P. For the shielded diode EDP-10, kNR varies from unity up to 20%, while the unshielded diode EDD-5 shows deviations up to 60% in the peripheral region. Thereby, the restricted application field of unshielded diodes has been clarified. For small field dosimetry purposes kNR can be converted into kNCSF, the non-calibration condition correction factor normalized to unity for a 4×4 cm2 calibration field. For the unshielded Si diodes needed in small-field dosimetry, the values of kNCSF are closer to unity than the associated kNR values.

A simplified approach to characterizing a kilovoltage source spectrum for accurate dose computation

To investigate and validate the clinical feasibility of using half-value layer (HVL) and peak tube potential (kVp) for characterizing a kilovoltage (kV) source spectrum for the purpose of computing kV x-ray dose accrued from imaging procedures. To use this approach to characterize a Varian® On-Board Imager® (OBI) source and perform experimental validation of a novel in-house hybrid dose computation algorithm for kV x-rays. We characterized the spectrum of an imaging kV x-ray source using the HVL and the kVp as the sole beam quality identifiers using third-party freeware Spektr to generate the spectra. We studied the sensitivity of our dose computation algorithm to uncertainties in the beam's HVL and kVp by systematically varying these spectral parameters. To validate our approach experimentally, we characterized the spectrum of a Varian® OBI system by measuring the HVL using a Farmer-type Capintec ion chamber (0.06 cc) in air and compared dose calculations using our computationally validated in-house kV dose calculation code to measured percent depth-dose and transverse dose profiles for 80, 100, and 125 kVp open beams in a homogeneous phantom and a heterogeneous phantom comprising tissue, lung, and bone equivalent materials. The sensitivity analysis of the beam quality parameters (i.e., HVL, kVp, and field size) on dose computation accuracy shows that typical measurement uncertainties in the HVL and kVp (±0.2 mm Al and ±2 kVp, respectively) source characterization parameters lead to dose computation errors of less than 2%. Furthermore, for an open beam with no added filtration, HVL variations affect dose computation accuracy by less than 1% for a 125 kVp beam when field size is varied from 5 × 5 cm(2) to 40 × 40 cm(2). The central axis depth dose calculations and experimental measurements for the 80, 100, and 125 kVp energies agreed within 2% for the homogeneous and heterogeneous block phantoms, and agreement for the transverse dose profiles was within 6%. The HVL and kVp are sufficient for characterizing a kV x-ray source spectrum for accurate dose computation. As these parameters can be easily and accurately measured, they provide for a clinically feasible approach to characterizing a kV energy spectrum to be used for patient specific x-ray dose computations. Furthermore, these results provide experimental validation of our novel hybrid dose computation algorithm.

An improved physics-based approach for unfolding megavoltage bremsstrahlung spectra using transmission analysis

To develop a physics-based approach to improve the accuracy and robustness of the ill-conditioned problem of unfolding megavoltage bremsstrahlung spectra from transmission data. Spectra are specified using a rigorously-benchmarked functional form. Since ion chambers are the typical detector used in transmission measurements, the energy response of a Farmer chamber is calculated using the EGSnrc Monte Carlo code, and the effect of approximating the energy response on the accuracy of the unfolded spectra is studied. A proposal is introduced to enhance spectral sensitivity by combining transmission data measured with multiple detectors of different energy response and by combining data from multiple attenuating materials. Monte Carlo methods are developed to correct for nonideal exponential attenuation (e.g., scatter effects and secondary attenuation). The performance of the proposed methods is evaluated for a diverse set of validated clinical spectra (3.5-25 MV) using analytical transmission data with simulated experimental noise. The approximations commonly used in previous studies for the ion-chamber energy response lead to significant errors in the unfolded spectra. Of the configurations studied, the one with best spectral sensitivity is to measure four full transmission curves using separate low-Z and high-Z attenuators in conjunction with two detectors of different energy response (the authors propose a Farmer-type ion chamber, once with a low-Z, and once with a high-Z buildup cap material), then to feed the data simultaneously to the unfolding algorithm. Deviations from ideal exponential attenuation are as much as 1.5% for the smallest transmission signals, and the proposed methods properly correct for those deviations. The transmission data with enhanced spectral sensitivity, combined with the accurate and flexible spectral functional form, lead to robust unfolding without requiring a priori knowledge of the spectrum. Compared with the commonly-used methods, the accuracy is improved for the unfolded spectra and for the unfolded mean incident electron kinetic energy by at least factors of three and four, respectively. With simulated experimental noise and a lowest transmission of 1%, the unfolded energy fluence spectra agree with the original spectra with a normalized root-mean-square deviation, %Δ(ψ), of 2.3%. The unfolded mean incident electron kinetic energies agree, on average, with the original values within 1.4%. A lowest transmission of only 10% still allows unfolding with %Δ(ψ) of 3.3%. In the presence of realistic experimental noise, the proposed approach significantly improves the accuracy and robustness of the spectral unfolding problem for all therapy and MV imaging beams of clinical interest.

A technique for calibrating a high dose rate 192Ir brachytherapy source

The reference air kerma rate of an ¹⁹²Ir high dose rate brachytherapy source is determined based broadly on the International Atomic Energy Agency (IAEA) TECDOC 1274 code of practice. Since the primary standards dosimetry laboratory at the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) does not maintain a standard at ¹⁹²Ir quality, the air kerma calibration coefficient of an IBA FC65-G Farmer type ionisation chamber is calculated using coefficients determined at 300 kV and ⁶⁰Co qualities. The methodology proposed by Mainegra-Hing and Rogers [1] at 250 kV and ¹³⁷Cs qualities is used. The validity of this approach is tested by performing Monte Carlo simulations to determine the chamber's air kerma calibration coefficient at ¹⁹²Ir quality. Very good agreement is obtained between values using these two methods. The reference air kerma rate is measured using the Farmer chamber in an in air jig. In addition the necessary correction factors are applied to the measured value. The reference air kerma rate determined in this way is compared to the value stated by the vendor of the ¹⁹²Ir source on the source calibration certificate. Differences are with one exception less than 1%. It is concluded that because of the agreement between the values from the methodology used in this study and the source calibration certificate values this methodology can be used clinically.

A simplified analytical approach to estimate the parameters required for strength determination of HDR 192Ir brachytherapy sources using a Farmer-type ionization chamber

Measuring the strength of high dose rate (HDR) (192)Ir brachytherapy sources on receipt from the vendor is an important component of a quality assurance program. Owing to their ready availability in radiotherapy departments, the Farmer-type ionization chambers are also used to determine the strength of HDR (192)Ir brachytherapy sources. The use of a Farmer-type ionization chamber requires the estimation of the scatter correction factor along with positioning error (c) and the constant of proportionality (f) to determine the strength of HDR (192)Ir brachytherapy sources. A simplified approach based on a least squares method was developed for estimating the values of f and M(s). The seven distance method was followed to record the ionization chamber readings for parameterization of f and M(s). Analytically calculated values of M(s) were used to determine the room scatter correction factor (K(sc)). The Monte Carlo simulations were also carried out to calculate f and K(sc) to verify the magnitude of the parameters determined by the proposed analytical approach. The value of f determined using the simplified analytical approach was found to be in excellent agreement with the Monte Carlo simulated value (within 0.7%). Analytically derived values of K(sc) were also found to be in good agreement with the Monte Carlo calculated values (within 1.47%). Being far simpler than the presently available methods of evaluating f, the proposed analytical approach can be adopted for routine use by clinical medical physicists to estimate f by hand calculations.

WE‐E‐BRB‐03: Extracting Energy Fluence Distributions of X‐Rays Produced by Megavoltage Electron Beams Stopping in Thick Targets from Lateral Profiles Measured Using Ionization Chambers

Purpose: To develop a method of extracting energy fluence distributions of x‐rays produced by megavoltage electron beams stopping in thick targets from lateral profiles measured using ionization chambers. Methods: High accuracy x‐ray lateral profiles measured using various ionization chambers with a set of build‐up caps for a number of target materials were recently reported. It was possible to extract energy fluence distributions from these profiles by using Monte Carlo calculations. The conversion factor was calculated as the ratio between the MC calculated energy fluence (using FLURZnrc) and the MC calculated absorbed dose in the air cavity of the ionization chamber (using “cavity”) at each position. The “measured” energy fluence lateral distribution was then obtained by multiplying the measured absorbed dose with these coefficients point by point, for each build‐up cap. The geometry, beam and material properties were simulated with BEAMnrc. The “cavity” user code was used to score the absorbed dose in the ionization chamber air cavity per incident electron up to angles of 25°. The energy fluence calculations were performed using the EGSnrc user code FLURZnrc. Results: The conversion factors for the PMMA build‐up caps were different by up to 40 % from those for the hevimet caps. However, when converting to energy fluence, for all targets the agreement between the energy fluence distributions obtained with each of the six build‐up caps was better than 1 %. The final energy fluence distribution for a particular target was obtained by averaging the distributions obtained for all six build‐up caps. Conclusions: The present study showed that it is possible to extract energy fluence distributions from x‐ray lateral in‐air profiles measured using commonly used Farmer type ionization chambers with various build‐up caps. The uncertainty on the distributions was better than 1 %.Partial support from NIH R01 CA104777‐01A2.

SU‐E‐T‐148: The IMRT and VMAT QA Experience Using PTW 2D‐ARRAY

Purpose: Accurate dose delivery is a key component in the success of radiation therapy. The dose verification is especially important in the high complexity treatments, such as Intensity Modulated Radiation Therapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT). Our purpose was to evaluate the dosimetric performance of our newly installed Elekta Synergy linear accelerator and Philips Pinnacle3 Planning system. Methods: The PTW Seven29 2D‐ARRAY consists of 729 vented ionization chambers with a dedicated phantom called Octavius were used to acquire the 2D dose distributions for the IMRT and VMAT plans. One limitation of this 2D‐ARRAY is the significant angular dependency, so we had measured thecorrection factors for VMAT plans. Another dosimetry system consists of a farmer type ion chamber (Wellhofer FC‐65P) and solid water phantom was used to measure the selected point dose. Total of 6 IMRT and 3 VMAT plans were included in this study. IMRT plans were verified with perpendicular deliveries at 0 degree gantry angle. VMAT plans were measured with actual treatment arcs, and then the corrections were applied after the measurements. We performed gamma comparison with 3% and 3mm criteria to evaluate the results of all 2D dose maps. Results: Our results showed good agreements in chamber dose validations. The differences between calculated and measured doses ranged from 0.7% to 3.1% (Mean 1.7%, SD 0.9%). The mean gamma index of 2D comparison from IMRT plans was 96.4% +/− 2.7% (SD). The mean gamma index of 2D comparison from VMAT plans was 95.1% +/− 2.8% (SD). Conclusions: Our preliminary evaluation has given us a satisfactory result, but the number of plans we have verified is just too small to give us enough confidence for all the situations we may encounter. We shall continue to validate the dosimetric performance of our planning and delivery system.

SU-E-T-87: Radiation Detector Responses to Applied Homogeneous Magnetic Fields

Purpose: To determine the relative dose deposition in a diamonddetector and an ion chamber from a clinical photon beam in an environment having varying magnetic fields, in an effort to evaluate dosimetry techniques in an integrated MR‐linac system. Methods: Using the Monte Carlo code PENELOPE, the PTW 60003 diamonddetector was modeled and irradiated with a 6MV photon beam spectrum (Varian 600C) in the presence of a homogeneous magnetic field. The distance from the source to the centre of the diamonddetector active volume was 100 cm and the detector was oriented perpendicular to the homogeneous magnetic field. An ion chamber (PR06) was also simulated with its long axis both parallel and perpendicular to the impinging photon beam, but perpendicular to the magnetic field in both cases. The homogeneous magnetic field was varied from 0 to 2500 gauss. The above system was also replicated experimentally with the aid of an electromagnet, and the measured relative ion chamber response as a function of magnetic field was compared with the simulations. Results: There is a significant variation (1.8% at 2170 Gauss) in the relative response of ion chamber as a function of magnetic field. Simulated ion chamber response follows the measured response closely. In contrast, both simulated and measured results indicate that the diamonddetector is relatively insensitive to the magnetic field compared to the farmer type ion chambers. Conclusions: This work has significant impact on dosimetry protocols for integrated MR‐linac systems, where the response of ion chambers is significantly affected by the presence of magnetic field. Thus, the use of ion chamber for reference dosimetry will require magnetic field dependent correction while a diamonddetector can safely be used as a beam scanning device.

An innovative method for 192Ir HDR calibration by farmer chamber, V-film, and solid phantom

A simple, practical and economical technique was proposed to calibrate an 192Ir HDR brachytherapy source in terms of air kerma strength. This technique makes use of the 0.6 cm 3 Farmer type ion chamber, radiographic film and polystyrene phantom. These tools are commonly used for dosimetry quality assurance of the clinical linear accelerator. In this study, the Exradin A19, PTW N30004 and TM30001 Farmer type ion chambers were used for the calibration of the 192Ir HDR source. To perform the calibration, a 25.4×30.5 cm 2 radiographic film was taped on a piece of polystyrene plate, and a straight applicator probe of a HDR brachytherapy unit and the Farmer type ion chamber were affixed to the film envelope. The film was irradiated by the 192Ir source, followed by an exposure in the simulator X-ray beam. The film set with the film removed was then placed on a 5 cm thick polystyrene phantom for calibration measurement. Based on the electrometer reading from the Farmer type ion chamber irradiated by 192Ir and the measured source-to-chamber distance by means of the images on the developed film, we can calculate the air kerma strength of the 192Ir using the new technique. Our calibration results were compared to the data provided by the manufacturer and that of five different well type ion chambers, namely, Sun Nuclear cooperation (SNC) 1008, Nucletron SDS 077.091, SDS 077.094, PTW TN33004 and Standard Imaging (SI) HDR-1000 Plus. The differences were all within 1.6%. Relative to the “7-distance measurement technique” by Stump et al., 2002, our method is more efficient if our empirical formula was used. In summary, our method is simpler and cost-effective to calibrate an 192Ir HDR brachytherapy source for those hospitals without a calibration jig or a well type ion chamber.

Air kerma strength characterization of a GZP6 Cobalt-60 brachytherapy source

Background Task group number 40 (TG-40) of the American Association of Physicists in Medicine (AAPM) has recommended calibration of any brachytherapy source before its clinical use. GZP6 afterloading brachytherapy unit is a 60Co high dose rate (HDR) system recently being used in some of the Iranian radiotherapy centers. Aim In this study air kerma strength (AKS) of 60Co source number three of this unit was estimated by Monte Carlo simulation and in air measurements. Materials and methods Simulation was performed by employing the MCNP-4C Monte Carlo code. Self-absorption of the source core and its capsule were taken into account when calculating air kerma strength. In-air measurements were performed according to the multiple distance method; where a specially designed jig and a 0.6 cm 3 Farmer type ionization chamber were used for the measurements. Monte Carlo simulation, in air measurement and GZP6 treatment planning results were compared for primary air kerma strength (as for November 8th 2005). Results Monte Carlo calculated and in air measured air kerma strength were respectively equal to 17240.01 μGym 2 h −1 and 16991.83 μGym 2 h −1. The value provided by the GZP6 treatment planning system (TPS) was “15355 μGym 2 h −1”. Conclusion The calculated and measured AKS values are in good agreement. Calculated-TPS and measured-TPS AKS values are also in agreement within the uncertainties related to our calculation, measurements and those certified by the GZP6 manufacturer. Considering the uncertainties, the TPS value for AKS is validated by our calculations and measurements, however, it is incorporated with a large uncertainty.

Flattening filter free beams in SBRT and IMRT: Dosimetric assessment of peripheral doses

Purpose Recently, there has been a growing interest in operating medical linear accelerators without a flattening filter. Due to reduced scatter, leaf transmission and radiation head leakage a reduction of out-of-field dose is expected for flattening filter free beams. The aim of the present study was to determine the impact of unflattened beams on peripheral dose for advanced treatment techniques with a large number of MUs. Material and methods An Elekta Precise linac was modified to provide 6 and 10 MV photon beams without a flattening filter. Basic beam data were collected and implemented into the TPS Oncentra Masterplan (Nucletron). Leakage radiation, which predominantly contributes to peripheral dose at larger distances from the field edge, was measured using a Farmer type ionisation chamber. SBRT (lung) and IMRT (prostate, head&neck) treatment plans were generated for 6 and 10 MV for both flattened and unflattened beams. All treatment plans were delivered to the relevant anatomic region of an anthropomorphic phantom which was extended by a solid water slab phantom. Dosimetric measurements were performed with TLD-700 rods, radiochromic films and a Farmer type ionisation chamber. The detectors were placed within the slab phantom and positioned along the isocentric longitudinal axis. Results Using unflattened beams results in a reduction of treatment head leakage by 52% for 6 and 65% for 10 MV. Thus, peripheral doses were in general smaller for treatment plans calculated with unflattened beams. At about 20 cm distance from the field edge the dose was on average reduced by 23 and 31% for the 6 and 10 MV SBRT plans. For the IMRT plans (10 MV) the average reduction was 16% for the prostate and 18% for the head&neck case, respectively. For all examined cases, the relative deviation between peripheral doses of flattened and unflattened beams was found to increase with increasing distance from the field. Conclusions Removing the flattening filter lead to reduced peripheral doses for advanced treatment techniques. The relative difference between peripheral doses of flattened and unflattened beams was more pronounced when the nominal beam energy was increased. Patients may benefit by decreased exposure of normal tissue to scattered dose outside the field.

SU‐GG‐T‐277: Dosi Metric Comparison of the New Siemens 160 MLC and the Varian Millennium 120 MLC

Purpose: Recent models of multileaf collimators (MLC) have smaller leaf widths and increased target conformality. The new Siemens 160 MLC and the Varian Millennium 120 MLC models both have individual leaf widths of 5 mm projected at isocenter. This study quantifies and compares the physical penumbra and transmission of the Siemens 160 MLC and the Varian 120 MLC for 6 and 10MV photon beams. Method and Materials: Penumbra was measured for 5×5 cm2 and 10×10 cm2 field sizes both on‐axis and off‐axis at depths ranging from 0.5 cm to 10 cm depth. Measurements were performed using the new Sun Nuclear Edge™ diode detector in a Scanditronix‐Wellhoffer scanning tank and confirmed with Kodak EDR2 film. The average of interleaf and intraleaf transmission and end leaf transmission were measured with Kodak EDR2 film and confirmed with a 0.6cc farmer type ion chamber. The films were scanned using a Vidar VXR 16‐bit scanner. The penumbra was determined for inplane and crossplane profiles. Results: The average central and off‐axis penumbra was 4.0 mm and 4.9 mm for the Siemens MLC and 4.3 mm and 4.5 mm for the Varian MLC, respectively. The penumbra increases with energy, field size, and depth for both MLC designs. The average of interleaf and intraleaf transmission was determined to be 0.7% for the Siemens MLC and 1.9% for the Varian MLC. End leaf transmission averaged 19.1% for the Siemens MLC and 30.3% for the Varian MLC. Conclusion: The Siemens MLC demonstrates a sharper off‐axis penumbra. while the Varian MLC demonstrates a sharper off‐axis penumbra. Penumbra for the Siemens MLC is on average 0.5 mm sharper on one side of the inplane profile. Due to the design and role of the Siemens MLC as a primary collimator, transmission was less through the Siemens MLC leaves than the Varian MLC.

SU-GG-T-611: Investigation of Different Carbon Fiber Tabletops Used in Radiation Therapy

Purpose: The dose changes in the buildup region and beam attenuation by several carbon fiber tabletops were investigated for 6 and 10 MV photon beams. Method and Materials: Measurements were performed for 3 × 3, 10 × 10, and 25 × 25 cm2 field sizes. The surface dose and percentage depth doses (PDD) were measured using a Markus parallel plate ionization chamber. A Farmer type ionization chamber with appropriate buildup caps have been used for attenuation measurement at several gantry angles (from 0° to 180°). Oncor's couch (Siemens schematic G5499), Primus' couch (MED-TEC model IL3005), and CT's couch (MED-TEC model IL3302) have been studied. Results: For a 6 MV beam Oncor couch increases the surface dose from 16.8% to 77.6% for small fields. Attenuation varies from 1% to 4.7% for 180° and 120° gantry angles. At 6 and 10 MV beams respectively, Siemen's couch increases the dose from 17.8% to 43.2% and from 10.0% to 28.5%, the beam attenuation varies from 0.1% to 8.6% and 0.2% to 3.1% for 180° and 120° gantry angles. The increase of the surface dose for the CT couch is from 17.5% to 77.8% and 9.6% to 58.9% for 6 and 10 MV beams respectively, and beam attenuation varies from 1% to 4.7% and 1% to 2.4% for 180° and 120° gantry angles. Conclusion: The carbon fiber tabletops significantly decrease the skin sparring effect. Our data show that the surface dose increase in the buildup region is about the same for the three couches. The dosimetric effect of the tabletop may be higher, especially for intensity-modulated radiation therapy depending on the beam orientation. Attenuation should be considered and corrected such as any material under the patient at the treatment planning stage. Keywords: Carbon fibre tabletops, Oncor, Primus, CT, PDD

Quality Control Of High-Dose-Rate Brachytherapy: Treatment Delivery Analysis Using Statistical Process Control

Purpose: Statistical process control (SPC) is a quality control method used to ensure that a process is on-target and operating with minimal variation. The focus of this study is to determine if SPC is a viable technique for evaluating the proper operation of a high-dose-rate (HDR) brachytherapy treatment delivery system. Materials and Methods: Thomson-Nielsen MOSFET Dosimetry System (MOSFET model 502RD) was used as the dosimeter. MOSFET dosimeters (MD) were calibrated against a Farmer-type ionization chamber at a distance of 1.6 cm from 192Ir in water. A surrogate prostate patient was developed utilizing Vyse Ordance™ gelatin. Needle placement was performed according to our prostate HDR clinical protocol. A total of 10 MD were placed from prostate base to apex. Three detector trains of three MD each, spaced 2.5 cm apart, were inserted into two detector holders at the prostate periphery as well as in the urethra. The remaining MD was placed at the apex on the periphery. CT guidance was used to accurately position the first detector in each train at the base. The plan consisted of 12 needles with 129 dwell positions spaced 0.5 cm apart, delivering a prescribed peripheral dose of 200 cGy. Marks were placed on the needles at the template, and the distance from the template to the needle hub was recorded to allow accurate repositioning of each needle. Sixteen accurate treatment trials were delivered as planned. Subsequently a number of treatments were delivered with errors introduced. The errors were: wrong patient (i.e., a clinical plan for a different patient with 12 needles), wrong source calibration (3 & 7 day source decay inaccuracy), wrong sequence (i.e. two needles connected to incorrect location in turret), single needle displaced inferiorly 5mm, and entire implant displaced 2mm and 4mm inferiorly. Each MD constituted an evaluation subgroup size of one (bias correction factors d2 & d3 determined for n = 2 for consistency with established SPC methodology). The range was therefore determined by the difference between sequential treatment delivery measurements. Two process behavior charts (PBC), an average and range chart, were developed for each MD. Results: There were 4 false positives resulting from 160 measurements from 16 accurately delivered treatments. Generally speaking, data points resulting from accurately delivered treatments were well within PBC limits. The PBC indicate that measurements made at the base and apex were much more sensitive to treatment delivery errors. Each of the errors introduced was correctly identified by either the average or range PBC for measurements made at the base (2 out of 3 locations) and apex (all 4 locations). The PBC for measurements made at mid-gland (3 locations) were less sensitive to errors in treatment delivery. The only errors detected at these locations were “wrong patient” and “wrong source calibration.” Conclusions: SPC is a viable methodology to assess the quality of HDR treatment delivery. Further development is necessary to determine the most effective positioning of the MD for detecting potential treatment delivery errors.

Evaluation of delivered monitor unit accuracy of gated step‐and‐shoot IMRT using a two‐dimensional detector array

To overcome the problem of organ motion in intensity-modulated radiation therapy (IMRT), gated IMRT is often used for the treatment of lung cancer. In this study, the authors investigated the accuracy of the delivered monitor units (MUs) from each segment during gated IMRT using a two-dimensional detector array for user-specific verification purpose. The authors planned a 6 MV photon, seven-port step-and-shoot lung IMRT delivery. The respiration signals for gated IMRT delivery were obtained from the one-dimensional moving phantom using the real-time position management (RPM) system (Varian Medical Systems, Palo Alto, CA). The beams were delivered using a Clinac iX (Varian Medical Systems, Palo Alto, CA) with the Millennium 120 MLC. The MatriXX (IBA Dosimetry GmbH, Germany) was validated through consistency and reproducibility tests as well as comparison with measurements from a Farmer-type ion chamber. The authors delivered beams with varying dose rates and duty cycles and analyzed the MatriXX data to evaluate MU delivery accuracy. There was quite good agreement between the planned segment MUs and the MUs computed from the MatriXX within +/- 2% error. The beam-on times computed from the MatriXX data were almost identical for all cases, and they matched well with the RPM beam-on and beam-off signals. A slight difference was observed between them, but it was less than 40 ms. The gated IMRT delivery demonstrated an MU delivery accuracy that was equivalent to ungated IMRT, and the delivered MUs with a gating signal agreed with the planned MUs within +/- 0.5 MU regardless of dose rate and duty cycle. The authors can conclude that gated IMRT is able to deliver an accurate dose to a patient during a procedure. The authors believe that the methodology and results can be transferred to other vendors' devices, particularly those that do not provide MLC log data for a verification purpose.

The Bland–Altman analysis: Does it have a role in assessing radiation dosimeter performance relative to an established standard?

Bland–Altman analysis is used to compare two different methods of measurement and to determine whether a new method of measurement may replace an existing accepted ‘gold standard’ method. In this work, Bland–Altman analysis has been applied to radiation dosimetry to compare the PTW Markus and Roos parallel plate ionisation chambers and a PTW PinPoint chamber against a Farmer type ionisation chamber which is accepted as the gold standard for radiation dosimetry in the clinic. Depth doses for low energy x-rays beams with energies of 50, 75 and 100 kVp were measured using each of the ionisation chambers. Depth doses were also calculated by interpolation of the data in the British Journal of Radiology (BJR) Report 25. From the Bland–Altman analysis, the mean dose difference between the two parallel plate chambers and the Farmer chambers was 1% over the range of depths measured. The PinPoint chamber gave significant dose differences compared to the Farmer chamber. There were also differences of up to 12% between the BJR Report 25 depth doses and the measured data. For the Bland–Altman plots, the lines representing the limits of agreement were selected to be a particular percentage agreement e.g. 1 or 2%, instead of being based on the standard deviation ( σ ) of the differences. The Bland–Altman statistical analysis is a powerful tool for making comparisons of ionisation chambers with an ionisation chamber that has been accepted as a ‘gold standard’. Therefore we conclude that Bland–Altman analysis does have a role in assessing radiation dosimeter performance relative to an established standard.

Dose perturbations by two carbon fiber treatment couches and the ability of a commercial treatment planning system to predict these effects

To measure the effect of the treatment couch on dose distributions and to investigate the ability of a modern planning system to accurately model these effects. This work measured the dose perturbation at depth and in the dose buildup region when one of two treatment couches, CIVCO (formerly MED-TEC) or Medical Intelligence, was placed between a photon beam source (6, 10, and 18 MV) and the phantom. Beam attenuation was measured in the center of a cylindrical acrylic phantom with a Farmer type ion chamber at multiple gantry angles. Dose buildup was measured in Solid Water with plane parallel ion chambers (NACP-02 and PTW Markus) with the beam normal to both the phantom and couch surfaces. The effective point of measurement method as described [M. R. McEwen et al. "The effective point of measurement of ionization chambers and the build-up anomaly in MV x-ray beams," Med. Phys. 35(3), 950-958 (2008)] was employed to calculate dose in the buildup region. Both experiments were modeled in XiO. Images of the treatment couches were merged with images of the phantoms such that they were included as part of the "patient" image. Dose distributions calculated with superposition and fast superposition algorithms were compared to measurement. The two treatment couches have different radiological signatures and dissimilar water equivalent thicknesses (4.2 vs 6.3 mm.) Maximum attenuation was 7%. Both couches caused significant loss of skin sparing, the worst case showing an increase in surface dose from 17% (no couch) to 88% (with couch). The TPS accurately predicted the surface dose (+/-3%) and the attenuation at depth when the phantom was in contact with the couch. For the open beam the TPS was less successful in the buildup region. The treatment couch is not radio-transparent. Its presence between the patient and beam source significantly alters dose in the patient. For the most part, a modern treatment planning system can adequately predict the altered dose distribution.

A Radiation Detector Constructed with GSO Scintillator and Plastic Optical Fiber Combination for Remote Dosimetry of Gamma-Ray Fields

A fiber-optic dosimeter model is constructed with a small piece of Gd2SiO5 (GSO) scintillator optically attached to a low attenuating plastic optical fiber. The lights generated in the scintillator are transmitted through the fiber and read by a current-type photomultiplier tube (PMT). The dosimeter model was tested with two 60Co standard sources of 1.85 and 37 MBq by measuring the PMT current as a function of the source-to-detector distance. It was then tested in a 60Co irradiation chamber with an activity of ˜244.2 TBq. MCNPX simulations were performed for the source and dosimeter arrangements to calculate the deposited energy in the GSO scintillator. When tested with standard 60Co sources of 1.85 and 37 MBq, the dosimeter model did not produce satisfactory results. However, better results were obtained with the higher-activity source. In the test in a 60Co irradiation chamber of 244.2 TBq, the measured data well coincide with the MCNPX simulation results. In a direct comparison with a Farmer-type ion chamber, it is found the dosimeter readings can be simply converted to the air kerma doses by proper calibration.

Comparison of peripheral dose measurements using Ionization chamber and MOSFET detector

Background In radiation therapy, the peripheral dose (PD) – the dose outside the geometric boundaries of the radiation field – is of clinical importance. A metal oxide semiconductor field effect transistor (MOSFET) detector is used to estimate the peripheral dose. Aim The aim of this study is to investigate the ability of a MOSFET dosimetry system to accurately measure doses in peripheral regions of high energy X-ray beams. Materials & Methods The accuracy of the MOSFET system is evaluated by comparing peripheral region dose measurement with the results of standard ionization chamber measurements. Furthermore, the measurement of PD using a MOSFET detector helps us to keep the tolerance dose of any critical organ closer to the treatment field within the acceptable limits. The measurements were carried out using a 0.6 cc Farmer type ionization chamber and MOSFET 20 dosimetry system for field sizes ranging from 5 × 5 cm 2 to 20 × 20 cm 2 at three depths of 1.5 cm, 5 cm and 10 cm in a blue water phantom. PD were measured at distances varying from 1 cm to 30 cm from the field edges along the x axis for the open fields, with collimator rotation and with beam modifiers like 15 degree, 30 degree and 45 degree wedges. Results The results show a good agreement of measured dose by both methods for various field sizes, collimator rotation and wedges. Conclusion The MOSFET detector has a compact construction, provides instant readout, is of minimal weight and can be used on any surface.